This invention relates to both methodology and apparatus for the noninvasive determination of hydrogen ion concentration ([H.sup.+ ]), commonly reported as -log ([H.sup.+ ]) or pH in tissue.
Current methodology for blood pH measurement requires blood samples to be measured using a sophisticated blood analyzer equipped with electrodes to measure the desired analyte. Standard analyzer instrumentation (such as supplied by Ciba Corning, Abbot Laboratories and Radiometer) automates sample preparation, delivery, and measurement. After each sample is analyzed, the electrodes must be thoroughly washed to prevent protein buildup on the electrode surfaces, and the analyzer must be calibrated at a minimum of every two hours. Measurement of pH, along with other blood analytes (i.e., PCO.sub.2, [HCO.sub.3.sup.- ], PO.sub.2 and O.sub.2 sat.), can be made with these analyzers in approximately two minutes. However, due to the size and expense of commercially available blood analyzers, such equipment is kept in central locations in most hospitals, requiring the blood sample to be transported to the analyzer.
While an individual analysis can be made in a few minutes, for an individual patient (often critically ill) the process is far from continuous and, because it is invasive, not without discomfort or pain. First, arterial blood has to be withdrawn. Immediately after withdrawal, the sample is placed on ice to inhibit red blood cell metabolism, which metabolism would alter the sample's blood gas parameters and lead to an incorrect measurement of the patients blood gas values (including pH). The sample is then, typically, transported to the clinical chemistry laboratory in the hospital, where it is logged in. Next, the sample is then analyzed by conventional electrochemical techniques with the type of equipment identified above. Finally, the results are entered in the hospital computer and made available to the physician for interpretation. Thus, the analysis can require a significant period of time (approximately 30 minutes) during which patient status can change.
Localizing pH measurement to treatment rooms, with the use of reagentless measurement of pH, would allow for semi-continuous measurement. This decrease in turnaround time for pH measurement would, in turn, provide the clinician better evaluation of treatment methods. While such improvements in pH measurement could be realized with in-vitro optical methodologies, greater gains can be realized with the noninvasive, in-vivo methodology as disclosed herein.
Several groups have published papers on the spectroscopic effects of pH variation on blood, primarily related to the Bohr effect. See, S. K. Soni and L. A. Kiesow, "pH-Dependent Soret Difference Spectra of the Deoxy and Carbonmonoxy Forms of Human Hemoglobin and Its Derivatives," Biochemistry, 16, 1165-1170, 1977, wherein the authors reported seeing variation in the Soret absorption bands of hemoglobin (i.e., 350-400 nm) with pH. These spectral variations were ascribed to structural changes in the porphyrin of hemoglobin, caused by pH variation. Other researchers have found that the visible absorption bands (i.e., 400-550 nm) from oxyhemoglobin have a pH sensitivity. See P. D. Wimberley, N. Fogh-Andersen, O. Siggaard-Andersen, F. C. Lundsgaard and W. G. Zijistra, "Effect of pH on the Absorption Spectrum of Human Oxyhemoglobin: a Potential Source of Error in Measuring the Oxygen Saturation of hemoglobin," Clinical Chemistry, 34, 750-754, 1988. Wimberley and coworkers did not attribute these spectral changes to the known variation in oxygen affinity caused by pH variation (i.e., Bohr effect). Rather, the variations seen were attributed to changes in the surrounding globin structure of the hemoglobin molecules, giving rise to changes in the charge-transfer bands of the porphyrin.
In the mid-infrared region (i.e., from approximately 2500 nm-10,000 nm), work has been done to determine the effects of pH variation on the vibrational modes of hemoglobin. In-vitro experiments with pure hemoglobin have shown that certain vibrational modes of the globins, in particular, the sulfur-hydrogen (S--H) stretching vibration of two cysteinyl (.alpha.Cys104, .beta.Cys112) are strongly hydrogen bonded and are sensitive to slight conformational changes of oxyhemoglobin caused by pH variation. See S. E. Antri, O. Sire and B. Alpert, "Relationship Between Protein/Solvent Proton Exchange and Progressive Conformation and Fluctuation Changes in hemoglobin," Euro. J. Biochem., 191, 163-168, 1990. These S--H vibrational modes were used to study the quaternary and tertiary structure of the hemoglobin molecule. However, from a clinical standpoint the mid-IR region is not useful because of the very high absorption of these wavelengths by tissue (i.e., there is no return signal from the tissue).
The near infrared region (i.e., from approximately 500 to 2500 nm) has been an active area of research for those pursuing non-invasive detection of blood analytes. Significant research has been done in the areas of oxygen saturation and glucose monitoring. See: (1) D. M. Haaland, M. R. Robinson, G. W. Koepp, E. V. Thomas and R. P. Eaton, "Reagentless Near-infrared Determination of Glucose in Whole Blood Using Multivariate Calibration," Appl. Spec., 46, 1575-1578, 1992; (2) B. Chance, J. S. Leigh, H. Miyake, D. S. Smith, S. Nioka, R. Greenfeld, M. Finander, K. Kaufman, W. Levy, M. Young, P. Cohen, H. Yoshioka and R. Boretsky, "Comparison of Time-Resolved and -Unresolved Measurements of Deoxyhemoglobin in Brain," Proc. Natl. Acad. Sci., 85, 4971-4975, 1988; (3) M. A. Arnold and G. W. Small, "Determination of Physiological Levels of Glucose in an Aqueous Matrix with Digitally Filtered Fourier Transform Near-infrared Spectra," Anal. Chem., 62, 1457-1464, 1990; (4) Y. Mendelson and M. V. Solamita, "The Feasibility of Spectrophotometric Measurements of Arterial Oxygen Saturation from the Fetal Scalp Utilizing Noninvasive Skin-Reflectance Pulse Oximetry," Biomedical Instrumentation & Technology, May/June, 215-224, 1992; (5) U.S. Pat. No. 4,975,581 to Robinson, et al.; and (6) U.S. Pat. No. 5,492,032 to Robinson, et al. The lowered extinction coefficients (e.g., the size or intensities of the bands) and decreased scatter coefficients in the near-infrared region affords a much greater penetration depth (i.e., 1-10 mm) into human tissue than the visible or mid-infrared regions. In the near-infrared, absorbances are primarily due to overtone and combination bands of fundamental stretching and bending vibrations, although some low lying electronic transitions appear at the shorter wavelengths. The strongest near infrared bands will arise when there is a strong asymmetry in the fundamental mode, such as exists with N--H, O--H and C--H vibrational modes, thus making these species the primary absorbers in the near infrared.
The most significant prior art on spectrograph measurement of pH is U.S. Pat. No. 5,355,880 to Edward V. Thomas, Mark R. Robinson, David M. Haaland and Mary K. Alam titled "Reliable Noninvasive Measurement of Blood Gases" (hereinafter "Thomas et al.") discloses methods and apparatus for determining noninvasively and in vivo at least two of the five blood gas parameters (i.e., pH, PCO.sub.2, [HCO.sub.3.sup.- ], PO.sub.2 and O.sub.2 sat.) in a human. The noninvasive methodology disclosed includes the steps of: generating light at three or more different wavelengths in the range of 500 nm to 2500 nm; irradiating blood containing tissue; measuring the intensities of the wavelengths emerging from the blood containing tissue to obtain a set of at least three spectral intensities v. wavelengths; and determining the unknown values of at least two of pH, [HCO.sub.3.sup.- ], PCO.sub.2 and a measure of oxygen concentration. The determined values were found to be within the physiological ranges observed in blood containing tissues. The methodology disclosed also includes the steps of providing calibration samples, determining if the spectral intensities v. wavelengths from the tissue represents an outlier, and determining if any of the calibration samples represents an outlier. The determination of the unknown values was performed by at least one multivariate algorithm (e.g., PLS (partial least squares), PCR (principal component regression) and CLS (classic least squares) using two or more variables and at least one calibration model. Preferably, there is a separate calibration for each blood gas parameter being determined. The methodology can be utilized in a pulse mode and can also be used invasively. The apparatus disclosed by Thomas et al. includes a tissue positioning device, a source, at least one detector, electronics, a microprocessor, memory, and apparatus for indicating the determined values.
The rationale of Thomas et al. for using multivariate analysis for noninvasive blood gas determination is to enable accurate determination of blood gas parameters where the information content in the spectral domain utilized overlaps and where the infrared patterns for pH, PCO.sub.2, PO.sub.2 and [HCO.sub.3.sup.- ] are small or do not exist in the absence of interactions with water and other blood or tissue components. This is especially true for pH which does not exhibit a strong correlation in any specific region. Thomas et al. also observed that not only does water have exceptionally strong absorption bands in the near infrared region, but H.sup.+ and O.sub.2 have no absorption bands of their own in the near infrared.
Thomas, et al., disclose the measurement of arterial blood gases on a lamb through the use of a Si array detector and a grating spectrometer. The spectra acquired were over the 500-1000 nm range. Thomas, et al., at col. 28, II. 28-35, went on to state:
Although the data used to demonstrate proof of concept in the lamb study was recorded from 500 to 1000 nm, this is not the only frequency region of interest. Specifically, the region from 1000 nm to 2400 nm contains information on both hydrogen ion concentration and CO.sub.2, (E. Watson and E. H. Baughman, On-line analysis of caustic streams by near-infrared spectroscopy. Spectroscopy, Vol. 2, No. 1, pp. 44-48.
In the reference by E. Watson, et al., the spectroscopic variations due to the varying hydrogen ion levels are due to changes in the water absorption bands. In summary, Thomas, et al., observed that hydrogen ion, being an ion rather than a molecule, does not have infrared bands. However, hydrogen ions will bind to other species in solution that are infrared active, thus a correlation for pH can be based on secondary spectroscopic effects. In the wavelength region of 1000-2500 nm, Thomas, et al., do not specify the exact source of the spectroscopic information for pH measurements in blood and no figures were provided disclosing the spectroscopic bands of importance.
In this application, the dominant source of spectroscopic information for pH measurement in blood in the 1000-2500 nm is identified, figures are provided demonstrating the relevant spectroscopic features, and the source of spectroscopic information is found to be different than identified by E. Watson.
It is an object of the invention to determine pH both in blood in vitro and in vivo in human tissue, utilizing spectral data from histidine in the range of 1000-2500 nm.
It is an object of the present invention to determine pH, both in blood in vitro and in vivo in human tissue, utilizing the spectral data from histidine in one or more of the ranges: 1000-1300 nm; 1500-1820 nm; 2040-2380 nm; and 1300-2380 nm.
It is a further object of the present invention to determine pH, both in blood and in vitro and in vivo in human tissue, utilizing spectral data from histidine at, approximately, one or more of the following wavelengths: 1042 nm; 1111 nm; 1163 nm; 1600 nm; 2060-2115 nm; 2160 nm; 2225-2235 nm; and 2360 nm.
A further object of the present invention is to provide a methodology, and associated apparatus, for determining pH, in blood in vitro and in vivo in human tissue, with standard errors or prediction below 0.05 pH units for a pH range of 1 (i.e., 6.8-7.8).